WARSAW (Reuters) - President Barack Obama
assured Poland and its eastern European neighbors on Tuesday that the
U.S. commitment to their security was sacrosanct at the start of a
four-day trip meant to show U.S. resolve after the Russian intervention
in Ukraine.

The White House unveiled plans for a $1 billion initiative to send
more of its military to Europe on a temporary basis but stopped
short of promising to beef up its permanent presence as some of
Washington's allies are seeking. It said the United States would
review its force presence on the continent.

Speaking in an aircraft hangar at Warsaw airport where he met U.S.
airmen taking part in a joint program with the Polish air force,
Obama said U.S. commitments to Poland and the region were a
cornerstone of the United States' own security.

"As friends and allies we stand united together," said Obama, whose
two-day stay in Warsaw will include meetings with Ukrainian
President-elect Petro Poroshenko and other central and eastern
European leaders.

Obama is under pressure from critics at home, who say he is not
showing enough firm leadership on the world stage, and from some
NATO allies in eastern Europe who fear they may be the next targets
for Russian expansion and want more U.S. protection.

But Western powers must also strike a delicate balance, because a
big increase in NATO forces on Russia's borders could prompt
reciprocal steps from the Kremlin and spiral into an arms race.

NATO defense ministers meet in Brussels on Tuesday to look at long
term measures to strengthen alliance defenses in eastern Europe and
consider how to combat the tactics used by Russia in Ukraine.

NEW VIGILANCE

U.S. Secretary of State John Kerry, also in Warsaw, said how to
respond to the crisis in Ukraine would be a focus of Obama's
meetings in Poland.

“We are here today because this remains a new moment of challenge
for all of us,” Kerry told reporters.

“Events in Ukraine have unfortunately unleashed forces that we had
all hoped had been put away ... were behind us. So it requires new
vigilance and it requires clear commitment."

The military assistance proposed by the White House, called the
European Reassurance Initiative, is to include greater U.S.
participation in training and exercises, deploying U.S. military
planners, and more persistent naval deployments in the Black Sea and
Baltic Sea, on Russia's doorstep.

The White House said in a statement it would build the capacity of
Ukraine and two other Western-leaning countries on Russia's borders,
Georgia and Moldova. Obama would be seeking the support of the U.S.
Congress for the plan, it said.

"In addition to this initiative, we are reviewing our force presence
in Europe in the light of the new security challenges on the
continent," it said.

"These efforts will not come at the expense of other defense
priorities, such as our commitment to the Asia Pacific rebalance."

Obama's visit to Poland coincides with the "Freedom Day" anniversary
in Poland, which marks the holding of the country's first
partially-free elections 25 years ago.

Later in the week, Obama travels to Brussels for a meeting of Group
of Seven leaders and then to France for the commemoration of the
70th anniversary of the D-Day landings in World War II.